1. Field of Invention
The present invention relates, in general, to improving the production efficiency of subterranean wells and, in particular, to a device and method which automatically detects when a motor in an electrical submersible pump assembly (“ESP”) is backspinning.
2. Description of the Prior Art
Submersible pumps are often used in deep wells for pumping large volumes of liquid to the surface. Often, the pump assembly will be located several thousand feet into the well. The pump assembly normally includes a centrifugal pump, below which is mounted a large alternating current electrical motor for driving the pump.
For various reasons, pumps are often shut down and power to the motor is cut off. When the power to the electrical motor is cut off, the motor will continue to spin along with the pump in a forward direction for a period of time, then the motor will cease spinning in a forward direction and start backspinning as a column of liquid above the pump falls downward into the well. It is important that while the motor is backspinning, the pump is not turned back on as the sudden surge of power to the motor creates extremely high torque on the shaft and may cause the motor shaft to break. Thus, it is important to determine when a motor stops backspinning before restarting the pump.
The current method of detecting a backspinning motor is to monitor the voltage generated by the residual magnetism of the motor. This voltage is proportional to the motor shaft speed and disappears when the motor stops.